Laid-open European Patent Application No. 289,041 discloses that 4-biphenylyl p-tolyl ether having the formula (I): ##STR1## is useful as a sensitizer for use in heat-sensitive recording paper and that it can be prepared in 80% yield by reacting p-phenylphenol with p-bromotoluene in the presence of a copper powder and recovering the reaction product from a toluene extract.
According to Japanese Patent Publication No. 451/1981, 4-biphenylyl p-tolyl ether can be obtained in 76% yield by reacting p-phenylphenol with p-chlorotoluene in the presence of a catalyst mixture composed of a copper chloride compound, a copper powder and activated alumina and in the presence of dimethylacetamide.
Furthermore, Japanese Kokai Tokkyo Koho No. 257938/1986 discloses the use of 1,3-dimethyl-2-imidazolidinone or sulfolane as a polar solvent in producing m-phenoxybenzyl alcohol by reacting chlorobenzene with m-hydroxybenzyl alcohol in the presence of a copper compound catalyst and a base.
For the production of diaryl ethers, such as 4-biphenylyl p-tolyl ether, the so-called Ullmann reaction is known, according to which a hydroxyarene metal salt is reacted with an aryl halide in the, presence of a copper catalyst. In said Ullmann reaction, aryl bromides, which are highly reactive, are generally used as the aryl halide, as described in Laid-open European Patent Application No. 289,04. However, aryl bromides are expensive and their use in commercial-scale production is disadvantageous from the economical viewpoint. On the other hand, aryl chlorides are very inexpensive, although they are much inferior in reactivity to aryl bromides. Therefore, the use of aryl chlorides is preferred from the commercial viewpoint.
As described in Japanese Patent Publication No. 451/1981, however, a long reaction period is required and the yield is unsatisfactory from the commercial viewpoint even when dimethylacetamide is used as a promoter. While it is known that N,N-dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, hexamethylphosphoramide, diethylene glycol alkyl ethers and the like can serve as reaction promoters in the Ullmann reaction, the use of these in the reaction of p-phenylphenol and p-chlorotoluene has failed to give 4-biphenylyl p-tolyl ether in commercially satisfactory yields.
According to Japanese Kokai Tokkyo Koho No. 257938/1986, the desired yield increase can be attained only under very strictly specified conditions, namely by using 1,3-dimethyl-2-imidazolidinone or sulfolane as a solvent in large amounts selected properly in relation to the starting materials and removing the byproduct water azeotropically with one of the starting materials within an appropriate temperature range. It is difficult to apply such a complicated reaction procedure to a reaction involving different starting materials.
Accordingly, it is an object of the invention to provide a method of producing 4-biphenylyl p-tolyl ether in a commercially advantageous manner.